This year in the U.S., close to 800,000 people will suffer a stroke, making it the third leading cause of death nationwide. The major cause of death in patients with large cerebral infarctions is cerebral edema. Among those suffering large middle cerebral artery strokes, mortality rates approach 80% within the first week following the event. No consistently effective non-surgical therapy exists for the direct relief of cerebral edema. Because cerebral edema following stroke is driven by the movement of ions and water within diseased brain tissue, progress in the development of clinically effective treatments will depend upon a more complete understanding of the molecular mechanisms governing water and solute movement in brain tissue. The brain does not have a conventional lymphatic system, so metabolic waste is cleared from brain extracellular fluid by its transport along the basal lamina, a sheath that surrounds brain capillaries. Previous studies suggest that fluid flow along the basal lamina communicates with the subarachnoid CSF compartment, constituting a 'paravascular fluid transport pathway'that functions as a de facto brain lymphatic system. However, the mechanics and molecular basis of this paravascular fluid transport remain poorly characterized. I propose to utilize both conventional histological approaches, ex vivo electrophysiology and in vivo 2-photon microscopy to characterize the mechanics and molecular basis of this paravascular fluid transport pathway. I present preliminary data demonstrating that this paravascular fluid transport is directional, with fluid from the cortical surface moving inward along penetrating arterioles to the level of cerebral capillaries. In Specific Aim 1, I will determine whether inward paravascular transport follows penetrating arterioles while outward clearance of extracellular fluid follows venules. In Specific Aim 2 I will test whether this directional transport is driven mechanically by arterial pulsation. I also present preliminary data demonstrating that mice lacking the aquaporin-4 (AQP4) water channel exhibit reduced paravascular fluid transport. In Specific Aim 3, I will evaluate whether the activity of the astrocytic NKCC1 ion co-transporter and the AQP4 water channel facilitate directional water flux across perivascular endfeet, thus contributing to directional paravascular flux. The proposed studies will explore the mechanisms governing water and solute flow in the brain, in addition to their compromise under conditions of cerebral ischemia, and may have important clinical implications for the treatment of this condition. PUBLIC HEALTH RELEVANCE: The major cause of death from large strokes is cerebral edema, a swelling of brain tissue for which there is no direct and effective non-surgical treatment. Cerebral edema is a disruption of proper water movement in the brain, and a poor understanding of the way that water movement is regulated in the brain hampers efforts to develop effective treatments. Here, the aim is to define how water flows along the outside of blood vessels in the brain, and to determine if the stoppage of this flow contributes to cerebral edema following stroke.